Face Of Tungsten Research Articles

Dynamics of the Surface Morphology of a Tungsten Foil under Load

The dynamics of changes in the surface morphology of recrystallized tungsten foil under the action of uniaxial tension was studied in situ by the methods of low-energy electron diffraction, electron Auger spectroscopy, atomic force microscopy, scanning electron microscopy, and energy dispersive scattering. It was established that, before the rupture of the sample, the multifractality–monofractality transition consists of several stages: the narrowing of the multifractal spectra of the singularities of the surface, the rotation of the structural blocks of the dominant (112) tungsten face, and the transition of the lattice of the dominant face into a diffractive-disordered state.

A general phenomenological model for work function

A general phenomenological model is presented for obtaining the zero Kelvin work function of any crystal facet of metals and semiconductors, both clean and covered with a monolayer of electropositive atoms. It utilizes the known physical structure of the crystal and the Fermi energy of the two-dimensional electron gas assumed to form on the surface. A key parameter is the number of electrons donated to the surface electron gas per surface lattice site or adsorbed atom, which is taken to be an integer. Initially this is found by trial and later justified by examining the state of the valence electrons of the relevant atoms. In the case of adsorbed monolayers of electropositive atoms a satisfactory justification could not always be found, particularly for cesium, but a trial value always predicted work functions close to the experimental values. The model can also predict the variation of work function with temperature for clean crystal facets. The model is applied to various crystal faces of tungsten, aluminium, silver, and select metal oxides, and most demonstrate good fits compared to available experimental values.

Helium bubble formation on tungsten in dependence of fabrication method

In order to investigate the morphology change of tungsten due to helium plasma exposure, tungsten (W) materials fabricated in different processes, ITER grade W, ultra-fine grained W (W–0.8TiC), and powder metallurgy W after helium plasma exposure were observed by scanning electron microscope. In all the three cases, pin-holes are observed on the surface; moreover, the non-uniformity of the formation of helium bubbles and fiberform nanostructure was observed on ITER grade W. From the detailed analysis of the crystal orientation with the help of focused ion beam method (FIB) indicates that the initial growth of the nanobubbles may be slow on the (110) face of tungsten.

The potential energy surface for dissociation of N 2 on W( [formula omitted

Electronic total energy methods have been used to compute the potential energy surface for the dissociation of nitrogen molecules on the (1 0 0) face of tungsten. The dissociation is found to be activated, but there are also deep molecular chemisorption wells into which molecules may steer and trap. The molecular geometries in these precursor states agree well with experimental assignments. The dominant well is for the adsorption of a molecule oriented vertically on the atop site. An analysis of the bonding there indicates that the orbitals break into components largely located on either the inner or the outer N atom, rather than the standard Blyholder model. Dissociation paths are suggested based on several steps, passing through at least two molecular precursor states.

Size effect in photofield emission from ( [formula omitted]) tungsten

Photofield emission characteristics from (0 0 1) face of tungsten for p-polarized light are measured. By differentiation of the true photocurrent curves, after subtracting thermocurrents, energy distributions of electrons are determined. For different photon energies three groups of electrons are obtained and they fit well the maxima obtained theoretically. The regular spaced small maxima in each group treated as the final states, lie in the same energy range about 2 eV above the Fermi level and are interpreted as resonance states caused by the limited size of the microcrystal at the apex of the tungsten field emitter. Similar effect has been observed previously for a thin titanium microcrystal grown on a tungsten field emitter. This interpretation is supported here by model calculations of the density of states distributions at the surface region, performed for a one-dimensional semi-infinite system.

Temperature dependence of the work function of hafnium islands on tungsten

Two-dimensional islands of hafnium near {100{ faces of tungsten and their terraces were examined by field electron microscopy. The two-dimensional sublimation energy of the islands was found to be 6.1±0.5 eV. The field electron emission from the islands grows with temperature, the effect being considerably greater than theoretically expected. The negative temperature coefficient of ϕ, αT, for the islands was estimated at (4–6)×10−4 eV grad−1 or more, whereas for pure tungsten αT was found to be 2×10−5 eV grad−1.

Temperature effect on the field emission from zirconium islands on tungsten

The coefficients α T of the change in the work function with temperature are determined experimentally for two-dimensional islands of zirconium on and in the vicinity of the {001} face of tungsten. The values of α T are an order of magnitude higher than those typical for clean metals. The reason is a change in the adatom-adatom binding energy with increasing temperature.

The influence of surface reconstruction on the collective motion of adsorbed atoms

The effect of adsorbate-induced surface reconstruction on the motion of adsorbed particles is analyzed by means of Monte Carlo modeling. The reconstruction is assumed to be the order–disorder phase transition that is described by the two-position model introduced by A.V. Myshlyavtsev et al. [J. Chem. Phys. 92 (1990) 3909]. The lattice gas model is used to mimic the hydrogen adsorption on the (001) face of tungsten [H/W(001)], which presents a c(2×2) structure below the critical temperature, T c. The phase diagram of the real system is reproduced only at low coverages ( θ<0.25). One of the principal features of the model is that the critical temperature can either increase or decrease with coverage, depending on the relation of the parameters which describe the system. In addition, different ordered phases, such as c(2×2) and (2×1) can be present according to the characteristics of the metal–metal, J NN, adsorbate–adsorbate, ε NN, and adsorbate–metal, λ NN, interactions. Utilizing Monte Carlo simulations we characterize how the behavior of the chemical, D, the jump, D J, and the tracer, D *, diffusion coefficients depend on the different ordered phases predicted by the model. In order to calculate the chemical diffusion coefficient we apply the fluctuation and Kubo–Green methods. Surface diffusion is strongly anisotropic for the (2×1) ordered phase, indicating that it is facilitated in one direction and reduced in another.

Structure and composition of tungsten silicide thermal-field microprotrusions

The thermal-field microprotrusions that grow on the surface of a tungsten tip coated with silicon when the tip is heated in an electric field are investigated by a suite of field emission methods, including electron field emission, ion desorption microscopy, and the atomic-probe method. For Si coatings more than a few monolayers thick, microprotrusions are observed to grow in the field desorption regime when the tip is heated to temperatures T=1100–1200 K in an electric field with initial intensity F=5.7–8.6×107 V/cm. The field at which they evaporate is 1.2–1.8×108 V/cm. The set of moving spots (i.e., microprotrusions) forms rings whose collapse signals the dissolution of the thermal-field growths on the developed faces. The most interesting structures are the sharp microprotrusions that grow on the central facet of a {110} tungsten tip under certain conditions. Atomic-probe analysis of their composition reveals that they consist of tungsten trisilicide WSi3 with a monolayer surface skin whose composition is close to WSi2. The intense growth of these formations on an initially smooth closepacked {110} face of tungsten is evidence that reconstruction of the latter takes place under the influence of the strong field and the interaction with silicon.

Residual Gas Effect on Photofield Emission from Tungsten (111) Face

Using the photofield emission method, surface states and bulk excitations from clean and residual gas adsorbed (111) face of tungsten were measured. Residual gas shaded the surface states and diminished photoemission from the bulk excitations. Gas of the artificially increased background pressure 10 -8 Pa adsorbed during the measurement deforms the photocurrent—voltage characteristic; the optical excitations observed for clean surfaces are exhibited, diminished or lost. For the adsorbed (111) face the state related to the metal—gas surface has been observed. PACS numbers: 79.70.+q, 79.60.Dp, 73.20.Hb

A model of linear chain submonolayer structures: application to [formula omitted] and [formula omitted

We propose a lattice gas model to account for linear chain structures adsorbed on (112) faces of tungsten and molybdenum. This model includes a dipole-dipole interaction as well as a long-range indirect (oscillatory) interaction of the form ∼ cos(2 k F r + ϕ)/ r, where k F is the wavevector of electrons at the corresponding Fermi surface and ϕ is a phase shift. It is assumed that the structures are stabilized by an attractive indirect interaction along the chains. We have explicitly demonstrated that the periodic ground states strongly depend on a competition between the dipole-dipole and long-range indirect interactions. The effect of temperature in our model of linear chain structures is studied within the molecular-field approximation. The numerical results clearly show that for the dipole-dipole interaction only, all long-periodic linear chain phases are suppressed to low temperatures while phases with periods 2, 3, and 4 dominate the phase diagram. However, when the long-range indirect interaction becomes important, the long-periodic linear chain phases start to fill up the phase diagram and develop a high thermal stability. We have chosen model parameters in order to reconstruct a sequence of long-periodic phases (for coverages less than 0.5) as observed experimentally at T = 77 K for Li Mo(112) and Li W(112) . It would be interesting to verify our model and assumptions by checking experimentally the corresponding phase diagrams.

Unoccupied electronic resonances of Sc adsorbed on W(001) by k-resolved inverse photoemission.

Scandium adsorbed on the (001) face of tungsten has been studied using Auger-electron spectroscopy, low-energy electron diffraction, k-resolved inverse photoelectron spectroscopy, work-function measurements, and relativistic-electronic-structure calculations. We find that the work function of W(001) does not decrease monotonically as a function of scandium adsorption but reaches a minimum value of \ensuremath{\varphi}=3.25 eV at 50% coverage. For a complete monolayer, an order 1\ifmmode\times\else\texttimes\fi{}1 Sc overlayer is formed. Inverse photoemission in the isochromat mode was used to map the unoccupied energy levels of this overlayer along the \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{} M\ifmmode\bar\else\textasciimacron\fi{} symmetry direction of the surface Brillouin zone. Local-density-functional calculations using the muffin-tin orbital method were performed for a 1\ifmmode\times\else\texttimes\fi{}1 Sc overlayer on W and are compared with the experimental two-dimensional band structure.

Displacement disorder and reconstruction of the (001) face of tungsten

The reconstruction of the (001) border of tungsten is examined taking into consideration random static displacements of surface atoms in the high-temperature (1 × 1) phase. A microscopic model is proposed, in which the creation of the c(2 × 2) phase is described as a transition of the Jahn-Teller type and an ordering of static displacements. It is shown that displacement disorder induces instability of (001) tungsten with respect to reconstruction. The effect of a uniform electric field on a disordered reconstructing surface is examined. A possible reason is given for pronounced differences in the results of investigations of the structural conversion of the (001) face in tungsten when different experimental methods are used.

On the possible mechanism of the W(001) surface reconstruction with adsorbed hydrogen

Abstract The reconstruction of the (001) face of tungsten is treated as a Jahn-Teller transition connected with two narrow surface electronic bands along the Γ M lines. The surface periodic lattice distortion is found to be accompanied by the occurence of the hydrogen ordering in an adlayer with the result that the reconstruction temperature raises.

Investigation of self-diffusion on tungsten by field emission microscopy

The field emission method has been applied to an investigation of self-diffusion on the (001), (011), (111) and (112) crystal faces of tungsten. Activation energies for separation of edge atoms on the (001) and (011) planes were calculated. The resulting values are 1.19 ± 0.11 eV for the (011) and 1.35 ± 0.08 eV for the (001) planes.

Low-temperature studies of adsorption of gases on metals

A molecular beam method has been used to study the kinetics of oxygen adsorption at sample temperatures T s ⩾ 4.2 K and gas temperatures in the effusion source 78 ⩽ T g ⩽ 550 K on the (100) and (112) tungsten faces and at T g = 200 K on the (110) face of tungsten. Adsorption properties of tungsten faces manifest themselves in the kinetics of dissociative oxygen adsorption within the first layer. The second and third monolayer filling for the three investigated tungsten faces shows practically the same kinetics during oxygen adsorption into the low-energy states with binding energies of ∼ 0.13 and ∼ 0.08 eV and desorption temperatures T s ≃ 45 K and T s ≃ 27 K, respectively. The oxygen condensation with a binding energy of ∼ 0.07 eV and sublimation temperature T s ≃ 25 K extends to the fourth monolayer. From the experimental data on the T s influence upon the initial sticking coefficient S 0 for the three tungsten faces and also for the niobum (110) face investigated earlier at temperatures T s ⩾ 78 K and at T g = 200 K conclusions have been drawn concerning the mechanisms of the elementary processes during dissociative adsorption. Using the experimental data on the T g influence upon the initial sticking coefficient in the second layer the critical translational energy for capture of incident oxygen molecules has been determined ( E c ⩽ 0.06 eV).

He diffraction from W(112): A quantitative analysis.

Diffraction data of helium atoms from the (112) face of tungsten are analyzed. The interaction potential thus obtained is compared to the predictions of the present theoretical approaches.Received 9 February 1987DOI:https://doi.org/10.1103/PhysRevB.36.4428©1987 American Physical Society

Ethane hydrogenolysis catalyzed by W(100)

The ability of the (100) face of tungsten to catalyze ethane hydrogenolysis has been investigated. At 573 K, methane is formed with a specific activity of 0.01 molecules per surface tungsten atom per second (100 Torr H 2, 1 Torr C 2H 6), with no evidence of catalyst deactivation even after several hundred turnovers. Hydrogenolysis is approximately first-order in ethane and half-order in dihydrogen, and displays an apparent activation energy of 27 kcal/mol. Negligible activity is displayed by clean tungsten; the true catalyst appears to be a monolayer of carbide which is formed on the surface of the tungsten on exposure to the reactants.

Compared mobilities of Cs and Na on W(110) at low coverage

The surface diffusion coefficient and compressibility if Cs on the (110) face of tungsten are measured by the field emission fluctuation method for coverages of 0.32 and 0.63 × 10 14 atoms/cm 2. An activation energy of 0.42 eV and preexponential of 10 −4 to 10 −5 cm 2 s −1 for diffusion are obtained. The compressibility is about 10 −3 to 10 −2 time that of the perfect gas. Comparisons with previous results of Na on W(110) are made.

Adsorbate-induced shifts of electronic surface states: Cs on the (100) faces of tungsten, molybdenum, and tantalum.

The adsorption of cesium on the (100) faces of W, Mo, and Ta for coverages between 0 and 1 monolayer is studied by angle-resolved ultraviolet photoemission spectroscopy with use of synchro- tron radiation, by electron-energy-loss spectroscopy, and by low-energy electron diffraction. With increasing cesiation, the W(100) surface state at \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{} located 0.3 eV below the Fermi level is shifted by up to 1.0 eV to larger binding energies while remaining sharp and intense. A similar behavior is observed on Ta(100), whereas on Mo(100) the shift of 0.9 eV of this surface state is accompanied by a pronounced attenuation of its intensity. These experimental shifts are shown to be in excellent agreement with all-electron local-density-functional results obtained with the full-potential linearized augmented-plane-wave method for Cs monolayers on the W(100) and Mo(100) surfaces. Based on these ab initio results, the electronic origin of the shifts is understood by the formation of strongly polarized covalent bonds between the d-like surface states and the Cs 6s--derived valence states. It is argued that even at high Cs coverages, the main electron-energy-loss peaks, which are observed between 1 and 2 eV, could be interpreted as Cs 6s\ensuremath{\rightarrow}6p--like interband transitions rather than as surface-plasmon peaks.